Feature/event subprocess

[essweine]

This replaces the waiting task index with something simpler that relies on existing task spec methods to maintain a list of waiting tasks. It also allows for the creation of subworkflows to be created outside the normal flow. This makes it easier to handle collaborations (the existing solution was a terrible hack that I've long been ashamed of) as well as spawning event subprocesses that do not have parents in the regular flow. Taken together, these changes are one more step towards being able to execute a collection or related processes in a way that keeps them distinct from each other.

I've also deprecated refresh_waiting_tasks in favor of refresh_timers, since (1) I want to ultimately get rid of callback methods passed to workflow methods (they're duplicative of more flexible task spec methods) and (2) the only tasks that need to be refreshed are timers, as other events will be updated when events are caught.

The other potentially useful addition is a bpmn_start_event attribute on workflow specs. This made disconnected workflows a lot easier to create, but could be potentially useful to library users so I want to flag it; it is probably an easier way to know how a process can start than the parser provides.

[essweine]

Also, this adds support for event subprocesses, the ultimate reason for the work, but which I forgot to mention originally.

[essweine]

Merging this so OneFM can start using it.